Thermoplastic polyamides, such as nylon 6 and nylon 6,6, are a class of materials which possess a good balance of properties comprising good elongation, high strength, high energy to break and stiffness which make them useful as structural materials. However, thermoplastic polyamides are quite sensitive to crack propagation. Consequently, a major deficiency of thermoplastic polyamides is their poor resistance to impact and their tendency to break in a brittle rather than ductile manner, especially when dry.
In general, improvements in the impact resistance of thermoplastic resins have been achieved by incorporating a low modulus rubber. Moreover, good dispersion of the rubber phase as well as developing adhesion between the rubber and matrix contribute to efficient impact modification of these resins.
It is well known to those skilled in the art that hydrogenated block copolymers of styrene and butadiene possess many of the properties useful for impact modification of plastics. These low modulus rubber materials display a low glass transition temperature, a characteristic advantageous for optimum toughening at lower temperatures. Furthermore, these block copolymers contain little unsaturation which facilitates their blending with high processing temperature plastics without significant degradation of the elastomer phase.
Block copolymers are unique impact modifiers compared to other rubbers in that they contain blocks which are microphase separated over the range of applications and processing conditions. These polymer segments may be tailored to become miscible with the resin to be modified. Good particle-matrix adhesion is obtained when different segments of the block copolymer reside in the matrix and in the rubber phase. This behavior is observed when hydrogenated block copolymer of styrene and butadiene are blended with resins such as polyolefins and polystyrene. Impact properties competitive with high impact polystyrene are obtained due to the compatibility of polystyrene with the polystyrene endblock of the block copolymer. Other polyolefins are toughened due to enhanced compatibility with the rubber segment.
Although the hydrogenated block copolymers do have many of the characteristics required for plastic impact modification, these materials are deficient as impact modifiers for many materials which are dissimilar in structure to styrene or hydrogenated butadiene. In particular, significant improvement in the impact resistance of polyamides with the addition of these hydrocarbon polymers has not been achieved. This result is due to poor interfacial interaction between the blend components and poor dispersion of the rubber particles. Poor interfacial adhesion affords areas of severe weakness in articles manufactured from such blends which when under impact result in facile mechanical failure.
The placement of functional groups onto the block copolymer may provide sites for interactions with such polar resins and, hence may extend the range of applicability of this elastomer. Such interactions, which include chemical reaction, hydrogen bonding and dipole interactions, are a route to achieving improved interfacial adhesion and particle dispersion, hence improved impact modification of polar thermoplastics.
Many attempts have been made to improve the impact properties of polyamides by adding low modulus modifiers which contain polar moieties as a result of polymerization or which have been modified to contain polar moieties by various grafting techniques. To this end, various compositions have been proposed utilizing such modifiers having carboxylic acid moieties and derivatives thereof, for example, Epstein in U.S. Pat. No. 4,174,358; Saito et el. in U.S. Pat. No. 4,429,076; Hergenrother et el. in U.S. Pat. No. 4,427,828; and Shiraki et al. in U.S. Pat. Nos. 4,628,072 and 4,657,971.
Epstein discloses a broad range of low modulus polyamide modifiers which have been prepared by free radical copolymerization of specific monomers with acid containing monomers. Alternatively, Epstein discloses the modification of polymers by grafting thereto specific carboxylic acid containing monomers. The grafting techniques allowed for therein are limited to thermal addition (ene reaction) and to nitrene insertion into C--H bonds or nitrene addition to C.dbd.C bonds (ethylenic unsaturation). Though Epstein does disclose a broad range of polyamide modifiers, Epstein does not disclose or suggest the utilization of hydrogenated copolymers of alkenyl arenes and conjugated dienes nor, more particularly, modified selectively hydrogenated copolymers of alkenyl arenes and conjugated dienes as polyamide modifiers.
Saito et al. disclose polyamide compositions which contain a modified unsaturated aromatic vinyl compound/conjugated diene block copolymer as a polyamide modifier. The unsaturated block copolymer has been modified by grafting a dicarboxylic acid group or derivative thereof (e.g. anhydride moieties) at a point of ethylenic unsaturation via thermal addition (ene reaction). However, such modifiers and compositions containing same are deficient in that the weatherability and resistance to thermal deterioration are poor; and, therefore, the polymers and compositions have been used only in the fields where such properties are not required. Furthermore, it is also noted that the ene reaction is a reversible reaction.
Hergenrother et al. and Shiraki et al. also describe a polyamide composition containing a block copolymer similar to that of Saito et al. However, in order to improve the weatherability and resistance to heat aging, both partially hydrogenate the block copolymer in their respective blends to an ethylenic unsaturation degree not exceeding 20 percent of the ethylenic unsaturation contained in the block copolymer prior to hydrogenation. Once the block copolymer is partially hydrogenated, the block copolymer is modified by grafting a molecular unit containing a carboxylic acid group and/or a group derived therefrom (e.g. anhydride moieties). Hergenrother et al. disclose grafting via thermal addition (ene reaction) utilizing the available residual unsaturation in the block copolymer. As such, Hergenrother et al. retained the deficiencies associated with the reversibility of the ene reaction. On the other hand, Shiraki et al. utilized free radical initiators to perform the grafting therein.
Though Shiraki et al. were apparently able to produce a nylon 6,6 blend possessing an Izod impact strength value at room temperature of about 14 ft.-lb./in. (75 kg.-cm/cm) (see Table 16 therein), such blends were apparently obtained solely by increasing the graft functionality levels to levels in excess of 4.0% w maleic anhydride utilizing a base (partially hydrogenated) block copolymer containing a residual ethylenic unsaturation of 10%. Additionally, there is no indication that such a blend when molded failed in a ductile, as opposed to brittle, manner during impact test utilized therein (ASTM D-256). Shiraki et el. were only interested in residual ethylenic unsaturation in the base block copolymers therein to the extent of improving weatherability.
However, there is a need to further reduce the graft functionality levels of the modified block copolymers utilized to impact modify .alpha.,.omega.-polyamides so as to eliminate, or at least minimize, processing problems relating to clogged vacuum systems, particularly with extruder-based modified block copolymer manufacturing systems. The vacuum systems are required for personal safety due to the hazardous nature of the modifiers utilized. An additional advantage to reducing graft functionality levels is cost reduction related to a reduction in the quantity of modifier used and to the costs associated to procure and operate the above-mentioned vacuum systems.
Further research and experimentation on polyamide compositions similar to those of Shiraki et el. have yielded unexpected and significant impact property improvements. In particular, super-toughened .alpha.,.omega.-polyamide blend compositions of the present invention are produced by regulating the proportion of the modified and optional unmodified block copolymers therein together with the degree of functionalization of the modified block copolymer and, particularly and quite surprisingly, the degree of ethylenic unsaturation of the unmodified and modified block copolymers.
To those skilled in the art, the degree to which the grafting reaction and phase size reduction occur, thereby promoting interfacial adhesion, together with the distribution of the rubber within the blend typically contribute to impact toughening of the blend. The results herein demonstrate that maintaining these three factors, i.e., unmodified and modified polymer content, degree of functionality and degree of ethylenic unsaturation of the unmodified and modified block copolymers, within specified limits promotes covalent bonding between the modified block copolymer and the .alpha.,.omega.-polyamide. Furthermore, the block copolymers also become well distributed in the polyamide phase. The super-tough blend compositions embodying the present invention are unexpected and surprising, i.e., that the blend of the .alpha.,.omega.-polyamide, the modified block copolymer with low graft functionality levels and the optional unmodified block copolymer (within certain relative proportions of unmodified to modified block copolymers) is uniquely super-tough whereas the blends of the .alpha.,.omega.-polyamide with either the unmodified block copolymer alone or when the proportion of the unmodified block copolymer to modified block copolymer exceeds effective amounts for super-toughening have not been observed to possess super-tough properties.